A steering apparatus includes telescopic shafts such as an intermediate shaft and a steering shaft, each having a male shaft and a female shaft capable of transmitting rotational torque and connected to be relatively slidable in an axial direction. The intermediate shaft is required to have a telescopic function when joining a universal joint to a pinion shaft to be engaged with a rack shaft of a steering gear so that the intermediate shaft is temporarily compressed before being engaged and joined to the pinion shaft, and also to absorb relative displacement with respect to a vehicle body frame.
The steering shaft transmits a steering force of the steering wheel to vehicle wheels, and is required to have a telescopic function to adjust a position of the steering wheel in an axial direction in accordance with a physical size and a driving posture of a driver.
In recent years, due to improvements in rigidity of the entire vehicle body and driving stability, it became easier for drivers to feel backlash of a telescopic shaft in rotation direction when operating a steering wheel. Therefore, a telescopic shaft with less backlash in the rotation direction, low sliding resistance, and superior lubricity and durability is being desired.
For that reason, there is a telescopic shaft in which an outer circumference of a tooth surface of a male shaft is covered with a resin or the like having low sliding resistance, and after applying a lubricant, the male shaft is fitted into a female shaft. FIG. 6 is a cross-sectional view of a conventional male shaft and a female shaft fitted onto the male shaft, illustrating a surface pressure which is applied to a covered portion of the male shaft. As shown in FIG. 6, a conventional male shaft 16A (male spline shaft) is fitted into a female shaft 16B (female spline cylinder) such that it is possible to relatively slide in an axial direction and transmit rotational torque. Protruding teeth 51 of the solid male shaft 16A are coated with a covering portion 61 made of resin to reduce sliding resistance between the protruding teeth 61 and tooth grooves 41 of the female shaft 16B, and are fitted into the tooth grooves 41 such that a tooth surface of the tooth grooves 41 and the covering portion 61 have a small interference.
In this conventional telescopic shaft, as shown in FIG. 6, bending moments acting on both end portions of an engagement region of the protruding teeth 51 and the tooth grooves 41 in an axial direction are large, and thus surface pressures at the both end portions are high. Therefore, both end portions of the covering portion 61 in an axial direction are deteriorated, and a backlash may occur between the male shaft and the female shaft. Such a backlash may occur between the male shaft and the female shaft due to similar reason, even in a case where the covering portion 61 is not provided. Particularly, in a column assist type power steering apparatus, due to a couple of force generated at a universal joint, surface pressures acting on both end portions of an engagement region of protruding teeth 51 and tooth grooves 41 of an intermediate shaft in the axial direction are high, and the likelihood of backlash being generated between a male shaft and a female shaft is high.
In a telescopic shaft of Patent Document 1, a male shaft is covered with a resin layer containing a sheet-like filler such as mica, whereby deterioration of the resin layer is suppressed, and backlash between the male shaft and a female shaft is suppressed. However, Patent Document 1 does not consider surface pressures applied to the resin layer at both end portions of the male shaft in an axial direction.
A telescopic shaft of Patent Document 2 has a ball provided between a male shaft and a female shaft. In order to maintain a preload on the ball for a long time so that a play is less likely to occur even if it is used for a long time, a deformation facilitating portion, which is easy to be deformed, is formed in a given region of the female shaft in a circumferential direction so that the female shaft is easy to bend, whereby a stress applied on the female shaft or the ball is reduced. However, Patent Document 2 does not consider a difference in the surface pressure in an axial direction.